Semiconductor wafer manufacturing technology utilizes very sophisticated wafer processing procedures and complicated manufacturing systems to produce semiconductor chips. Each and every wafer goes through several steps, such as resist spinning, etching, dielectric layer depositions, metal depositions, and different encapsulation layers. These process steps are performed one or more times to define and etch pre-designed patterns on the silicon wafers. The wafers are subjected to several high temperature processes varying from 400.degree. C. to 1400.degree. C. It is during these multiple processing and deposition processes involving layers of different types of metals and die-electric that a great deal of stress is built up in the layers on the wafer due to differences in coefficients of expansion of the individual layers that are in direct contact with each other.
The ever increasing demand for reduced product sizes and miniaturization of computers has forced semiconductor manufacturers to shrink all parts going into electronic products. Common examples are Laptop Computers, Notebook Computers, Mobile Telephones, etc. In order to manufacture these products, manufacturers have to make smaller displays and key boards, and to produce thinner PC Boards. Manufacturers must also reduce the thicknesses of assembled IC packages so that the satisfy miniaturization parameters.
In order to meet the challenges of Thin Small Outline Packages (TSOP), semiconductor manufacturers have been forced to reduce the thickness of silicon wafers before sawing and dicing operations. Historically, post clean room wafer processing was done by using a backside wafer preparation technique to provide a good electrical contact to the substrate. After being processed through multiple oxidations and diffusions, the backside surface of the wafer was uncharacterized and unpredictable. A means of removing approximately 10-30 microns of material from the back of wafer was necessary.
Several methods have been used in the prior art to prepare the backside of the wafer for packaging. The dirtiest and probably the harshest process to clean the backside of silicon wafers was sand blasting. The same method used to clean building walls was used to clean silicon wafers. Cleaner technologies were developed by many device fabrication facilities that employed resist and plasma technologies. Some of these facilities tried chemical etching with a mixture of nitric acids, hydrofluoric acids, and acetic acids. All of these etching methods proved to be slow and not suitable for production mode. Moreover, the etching processes did not prove to be adding any significant advantages to the wafer, such as improving wafer strength, die strength, and electrical yields. It was not until a new market began to emerge that backgrind processing technology received industry attention. That new market was laptop computers.
Laptop computer technologies effectively pushed device-packaging technology users to meet a new set of dimensional demands. It has been posited by many market research analysts that in the future, integrated circuit manufacturers will be forced to employ higher and higher levels of multi-chip integration in order to maintain a profitable level of value added. It seems obvious that every part intended for a thin package will have to be reduced in thickness in order to accommodate the future multi-chip package schemes. The only viable alternative is therefore to backgrind each individual wafer after clean room processing.
The Backgrind Process
The first step in the backgrind process involves the application of a protective tape to protect the front side of the wafer. Several manufacturers such as Mitsui, Nitto Dhenko, Furukawa, Marubeni, and the like manufacture these tapes. These tapes are made of different types of base films coated with adhesives. Some of the adhesives are UV types and some are Non-UV types. Tape application machines are manufactured and sold by several companies such as Takatori, Nitto, Advantec, and the like.
Following application of the protective tape, the taped wafers are loaded onto wafer grinding systems. Fully automatic wafer grinding systems are manufactured by R. Howard Strasbaugh, Disco, Okamoto, and the like. These systems utilize sophisticated materials and mechanics to provide aggressive and uniform silicon removal. After the correct amount of material is removed from the back of the wafer, the tape is removed either manually or with an automatic tape removal system, and the wafers are cleaned.
When a wafer arrives in a backgrind process bay there exists a net cumulative film stress resulting from the thermal process stress induced during the manufacturing steps. The wafer is then thinned by the backgrind process. The stress resulting from the wafer grinding process adds to the thermal process stress already in the wafer. The cumulative stress built into the wafer leads to decreased wafer strength, subjecting the wafer to die cracking and other stress related failures in IC Packages, and more importantly, electrical failures of IC packages at Final Test. Grinding wafers with diamond wheels inevitably leaves flaws on the wafer back surface. These flaws, inclusions, often weaken both the wafer and the individual die after the wafers are sawed. Under thermal and mechanical stresses during packaging of IC's these inclusions can spread into active regions. Ultimately they may crack the entire die.
Accordingly, it is an object of the present invention to provide a process that reduces mechanical stress induced during the processing of wafers. This improves the mechanical strength of the full size silicon wafer, the electrical yields of the finished wafer at the wafer sort level, and reduces die cracking of the products in packages, thereby improving final test yields of the finished products after assembly and packaging.
It is a further object of the present invention to provide a process that removes microcracks within the wafer caused by wafer grinding.